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fkamumcy ATTORNEY Patented July 8, 1941 PHASE ivmDULA'rIoN Murray G. Crosby, Riverhead,ll\l.'Y.,"' assigner to Radio Corporation of America,` a corporation ofV Delaware Application February 12, 1941,'seria1N0. 378,517

8 claims.

'Ihis application describes a `type of phase modulator which makes use of the principle that a retard circuit produces an output with its phase proportional to the frequency'of the applied input. quency modulated wave is heterodyned ,and passed through the retard circuit so that a phase shift with frequency is superimposed upon the frequency modulated wave. This frequencyphase modulated Wave is then l heterodyned back in a manner that removes the frequency modulation component, but leaves the phase modulation component. By this means a phase modulator is provided which has high phase deviation capabilities and has a stability which isiny dependent of the stability of the frequency modulated oscillator used.

In my United States application Serial No. l727,931, led May 28, 1934, now Patent #2,042, 831 issued June 2, 1936, this principle of producing a phase shift by varying the input frequency of a retard circuit is disclosed as a means of controlling the phase of a Wave. In my United States application Serial No. 618,154, filed June 20, 1932, Patent No. 2,229,640, January 28, 1941, the same principle is used to produce a frequency modulation detecting system.

In describing my novel method and means for producing high deviation phase modulation, reference will be made to the attached drawing -4 wherein:

Fig. 1 shows by block diagram a phase modulation system arranged in accordance with my invention and including a frequency modulated oscillator, a stable oscillator to heterodyne therewith, a retard circuit through which the heterodyne currents are passed, and means for beating the latter with the frequency modulated oscillations to suppress the frequency modulation and derive the high deviation phase modulation output;

Fig. 2 is a somewhat detailed showing of a phase modulator such as shown by block diagram in Fig. 1;

Fig.. 3a is one form of retard circuit; and

Fig, 3b is a curve showing the manner in which the phase shift in accordance with frequency modulation is obtained in the phase retard circuit.

Fig. l shows a block diagram of the elements of the system. A stable oscillator l beats with frequency modulated oscillator 5 in detector 2 to produce a heterodyne output for application to the input of retard circuit 3. The heterodyned output will be frequency modulated the same In accordance with my disclosure, a frenumber of cycles deviation as is present on the oscillations produced in 5. 'I'he output phase at the output .of retard circuit 3 Will then vary in accordance with the frequency modulation. 'Ihe manner of thisphase variation is shown in Figs. 3a and 3b in which Fig. 3a is a band-pass lter type of retard circuit having a phase shift characteristic shown by Fig, 3b. In the characteristc shown in Fig. 3b, the phase shift imparted by the lter varies from -1r at the lower cut-oil? frequency, Fl, to zero at the upper cut-off frequency, F2. Normally the unmodulated carrier frequency is tuned midway between the two cutoi frequencies so that a phase deviation of plus or minus 1r/2 radians is obtained as the frequency is modulated between the two cut-olf frequencies.

`As an example, stable oscillator at l might have a frequency o-f 6 megacycles and the frequency modulated oscillator in 5 a carrier frequency of 5 megacycles. This produces a heterodyne frequency, to be fed to the retard circuit in 3, of one megacycle. When this frequencymodulated-one-megacycle wave is passed through the retard circuit at 3, a phase shift is imparted which is in accordance with the frequency of the wave passed. This frequencyphase modulated wave is then heterodyned, in detector 4, with rthe original frequency modulated wave from the modulated oscillator in 5. The sum frequency of (5+1) megacycles is chosen in the output of the detector at 4 so that the output frequency is the same as that of the stable oscillator in I. In this heterodyne process the frequency modulated component of the frequency-phase modulated wave is removed so that the output is a phase modulated wave with no instability produced by the presence of the relatively unstable frequency modulator in the circuit.

In this example, let us assume that the frequency modulated oscillator is frequency modulated to 5.1 megacycles. This causes the heterodyned output of the detector at 2 to be modulated from 1.0 to 0.9 megacycles. The 0.9 megacycle output of the retard circuit is then heterodyned with the 5.1 megacycle frequency-modulated oscillator so that the output frequency is 5.1-1-0.9 or 6.0 megacycles. Thus the frequency modulation is removed by this heterodyne arrangement.

Fig. 2 is a specific embodiment of my phase modulator in which the retard circuit consists of tuned circuit 20 connected between the anode 24 and cathode 26 of a multi-grid detector mixer tube 28 the grids 30 and 32 of which are excited by oscillations of constant frequency from oscillator 1 and frequency modulated oscillations from modulated oscillator tube 50 of the frequency modulator 5.

The frequency modulated oscillator consists of oscillator tube 50 and reactance tube 54. The tube 54 has its anode 55 coupled to the anode 58 of oscillator tube 50. The grid 60 of tube 54 is connected to the cathode by a resistance R and to the anodes 5B and 58 by a condenser C. C and R form a phase shifting circuit so that the high frequency voltage on the anode 56 is shifted in phase about 90 degrees with respect to the phase of the voltage on the control grid 6U. Due to this phase quadrature relation, a reactive effect is produced between the anode and cathode of the reactance tube 54. This reactive effect is in shunt to the tuned circuit 60 of the oscillator. 66 is a radio-frequency choke feeding plate voltage to anode 55.

The by-pass condenser 61 is connected between the low direct-current potential end of the choke 56 and the cathode. The operation of reactance tubes is now well known and has been described in detail in my United States application #136,- 578, filed April 13, 1937, and therefore it is believed unnecessary to describe the same in detail here.

Modulating potentials are supplied from any source to the primary winding of transformer 18 and from the secondary windingof transformer l!! to the control grid H of tube 54. Variations in the potential on the control grid 1| due to the modulation produces variations in the reactance of the tube 54 and these reactance variations modulate the frequency of the oscillations generated at 50.

'I'he stable oscillator l comprises a tube 8B having its anode and control grid connected in highfrequency oscillation generating circuits including a piezo-electric crystal 82 and a tuned anode circuit 84.

The oscillations produced in the stabilized oscillator l are mixed with the frequency modulated oscillations from 5 in detector and mixer tube 2 and the beat frequency, which is frequency modulated, is passed by way of the retard circuit 20 Aand coupling condenser 89 to the control electrode 9D of the detector and mixing tube 92. The grid 94 of this tube is also excited by frequency modulated oscillations from the modulated oscillation generator comprising tube 50 by way of lead 95.

This phase modulator may be converted to a frequency modulator by the insertion of a correcting network in the modulation input circuit. This correcting network would have yan 4output inversely proportional to the frequency of the applied modulating potentials. For this type of frequency modulation production, this modulator has an advantage over those of the prior art since large degrees of phase deviation may be produced by inserting several sections of band-pass filter to increase the degree of phase shift with frequency. This high phase shift at the modulator then removes the necessity of large degrees of frequency multiplication to produce the required degree of frequency modulation.

It will be understood that the retard circuit used in this modulator is not limited to the tuned circuit shown in Fig. 2 or the band-pass lter shown in Fig. 3. Any type of circuit which produces a phase shift with frequency at its output is suitable. The preferable circuit is a band-pass filter, but a low-pass or high-.pass may also be used. The ordinary tuned transformer with inductive coupling provides a simple band-passfilter type of retard circuit. The degree of phase deviation may be increased by inserting several stages of tuned transformers in cascade.

The principle of this circuit may be used as a means of receiving frequency modulation. This may be done by replacing the frequency modulated oscillator by pick-up and amplifying means responsive to an incoming frequency modulated signal. The output of detector 4 is then a phase modulated signal which is combined with local carrier from stable oscillator I so that the phase modulation may be detected to receive the frequency modulation on the incoming signal.

What is claimed is:

1. The method of converting wave length modulation having predominantly the characteristics of frequency modulation into wave length modulation having predominantly the characteristic of phase modulation which includes the steps of, beating a wave length modulated wave with a wave of substantially fixed frequency to derive other correspondingly modulated wave energy, passing said other wave energy through a path the electrical length of which varies with wave length variations of said other wave energy, and beating the wave energy passed by said path with said rst mentioned wave length modulated wave energy.

2. The method of converting wave length modulation having predominantly the characteristics of frequency modulation into wave length modulation having predominantly the characteristics of phase modulation which includes the steps of, beating said first mentioned wave length modulated wave with a wave of substantially xed frequency to derive other correspondingly modulated wave energy of a difference frequency, passing said derived other wave energy through a path the electrical length of which varies with wave length variations of said other wave energy, and beating the wave energy passed by the said path with said rst mentioned wave length modulated wave energy to derive resultant energy.

3. In apparatus for converting wave length modulated wave energy having predominantly the characteristics of frequency modulation into wave length modulated wave energy having predominantly the characteristics of phase modulation, means for heterodyning said first mentioned wave energy with wave energy of substantially constant frequency, means for passing the energy resulting from said heterodyning process through a circuit the electrical length of which varies with variations in frequency of the passed wave energy, to superimpose thereon modulation having the characteristics of phase modulated wave energy, means for mixing said last named wave energy with said first mentioned wave energy, and means for deriving wave energy having the characteristics of phase modulated energy from said last mixing process.

4. vIn an apparatus for converting high deviation wave length modulated wave energy having predominantly the characteristics of frequency modulation into high deviation modulated wave energy having predominantly the characteristics of phase modulation, a source of oscillations of substantially fixed frequency, heterodyning means coupled to said source, means for impressing said rst mentioned wave energy on said heterodyning means, a detector having an input coupled to said heterodyning means, means in said coupling the electrical length of which varies with Variations in frequency of Wave energy passed by said coupling, means for impressing said first mentioned wave energy on said detector, and means for utilizing resultant energy in said detector.

5. The method of producing wave energy the phase of which has been deviated through a Wide range by means of wave energy subjected to high deviation frequency modulation in accordance with signals which includes the steps of, heterodyning said frequency modulated Wave energy with oscillatory energy of substantially fixed frequency, deriving the diiference frequency of the heterodyned energies, retarding the phase of the derived energy in accordance with the frequency deviations thereof, beating the resultant energy with the original frequency modulated Wave energy, and deriving the sum frequency of the last beating process for signalling purposes.

6. A phase modulator comprising in combination, a source of wave energy modulated in frequency in accordance with signals, a source of Wave energy of substantially constant frequency, means for heterodyning Wave energy from both of said sources, means for passing the Wave energy resulting from said heterodyning through a circuit the electrical length of which varies with variations in frequency of the energy passed thereby, and means for heterodyning the Wave energy passed by said circuit with frequency modulated wave energy from said first source to derive a phase modulated output.

7. A phase modulator comprising in combination, a source of Wave energy modulated in frequency in accordance with signals, a source of Wave energy of substantially constant frequency, means for heterodyning Wave energy from both of said sources to derive Wave energy of a difference frequency, means for passing the Wave energy resulting from said heterodyning through a circuit the electrical length of which Varies with variations in frequency of the energy passed thereby, and means for heterodyning the Wave energy passed by said circuit with frequency modulated Wave energy from said first source to derive the sum frequency.

8. In a phase modulator, a source of oscillations of substantially fixed frequency, a source of oscillations of controllable frequency, means for controlling the frequency of said last source of oscillations in accordance with signals comprising a reactance tube coupled with said last source' of oscillations and with a source of control potentials, a mixer tube having input electrodes coupled to both of said aforesaid sources said mixer tube having output electrodes, an electrical circuit the length of which varies in accordance with applied wave frequency connected to the output electrodes of said mixer tube, a second mixer tube coupled with said last named circuit and with said oscillator of controllable frequency, and an output circuit coupled with said last named mixer tube.

MURRAY G. CROSBY. 

